1. Field of the Invention
The present invention relates generally to a process for removing elemental mercury from flue gases.
2. Description of the Related Art
Introduction
Mercury emissions are regarded as one of the worlds most problematic environmental issues, because of their propensity to bio-accumulate by up to a factor of 10,000 within an aquatic food chain. Mercury, after bioaccumulation, may result in neuron damage of human beings. It is estimated that 80% of total anthropogenic mercury emissions from 1994 to 1995 was from combustion; of which 33% was associated with coal-fired utility boilers. The United States Environmental Protection Agency (USEPA) announced its final regulations on mercury emissions from coal-fired utility boilers in March of 2005. This regulation stated that yearly mercury emissions are to be reduced by 20.8% by 2010 and 68.8% by 2018 from levels in 1999.
Mercury occurs in the flue gas of coal-fired utility boilers in three valence states, elemental Hg0, oxidized (Hg2+), and particle-bound (Hg(P)). There are known techniques for the effective removal of Hg2+ and Hg(P) from flue gases. However, Hg0 is relative stability, water insolubility, and has a long range transportation in the atmosphere as compared to Hg2+ and Hg(P). Thus, it is desirable to effectively convert Hg0 to Hg2+ or Hg0 and Hg2+ to Hg(P) so that the known removal techniques may be employed for the removal of Hg0 from flue gas.
The combined utilization of selective catalytic reduction (SCR) to oxidize the Hg0 into Hg2+ and the subsequent Hg2±removal by wet-flue gas desulphurization (wet-FGD) in coal-fired utilities is known. This method for controlling the emissions of SOx, NOx and Hg0 has proven effective when burning some coals such as bituminous coals, but simultaneous availability of both SCR and wet-FGD is limited in the united states (approximately 25%). Moreover, this method has not been shown to be effective when burning Powder River Basin (PBR) coal or lignite. Bituminous coals generally have a higher concentration of naturally occurring therein chlorine, sometimes comparable fluorine, which is thought to aid in the oxidation of Hg0. Even though coal likely has other halogens such as bromine and iodine, the concentrations are too low to effectuate the oxidation of the Hg0. Furthermore, presently prevailing Hg control technology is still necessary in Bituminous coal burning plants because of low affinity of mercury to the fly ash. It is indicated economics of mercury capture technology is directly relative to the percentage of Hg(P) in Hg(VT) because over 80% of coal-fired power plants are equipped with fly ash removal facilities, such as cold-side ESP (electric precipitator) and FF (fabric filter). Therefore, conversion of either Hg0 to Hg2+ and/or Hg0 and Hg2+ to Hg(P), in all coal-derived flue gas seems limited.